Face Perception

Monday, 20 February 2023 5:36 PM

Faces as unique objects

- It takes multiple layers of processing to process a face
○ Categorical (age, sex, race etc.)
○ Semantic (linking a face to a person's name)
- Humans are specialised at face processing
○ Evolutionary advantage to being able to quickly identify members of social groups
Models of facial processing
- Figural distinction
○ Face vs. non-face objects
- Semantic distinction
○ Race/sex
- Expression decoding
○ Emotions and intended behaviour

Evidence for specialised face processing

Perceptual evidence
- Face inversion effect
○ Facial recognition is hard to do when the face is flipped upside down, as compared to other upside-down objects
- Contrast inversion effect
○ Facial recognition is harder with reversed contrast of light/dark, as compared to other reverse contrasted objects
- Caricature enhancement
○ We can still recognise faces and identities of cartoon drawings
- Spatial alterations
○ Recognition of faces is fairly similar even after extreme deformations (stretching etc)
Behavioural evidence
- Behavioural evidence indicates that visual information about faces is processed differently to other objects
○ i.e. holistic processing
- Face inversion effect
○ upright faces are processed holistically vs. inverted faces are processed figurally
○ It's easier to perceive a face that is the right way up than it is upside-down
- The composite effect
○ Occurs when participants are shown the top half of one face aligned with the bottom half of another face
▪ subjects are slower to identify half of a chimeric face aligned with an inconsistent other half-face
▪ Subjects are quicker to identify the half faces if they are misaligned, rather than aligned
○ This recognition difficulty indicates obligatory holistic processing of faces
▪ We can't pick and choose what we process from a face, we process everything all at once rather than each component separately
▪ We cannot single out specific elements, e.g. half of a face at a time
○ However, when faces are misaligned facial processing is a lot quicker
▪ Because we can process them separately instead of together
- The face composite illusion
○ The perception of the top half of the face is influenced by features in the bottom half
▪ We perceive the top half of a face to differ between examples with different bottom halves even when the top half is the same
▪ Indicates holistic processing
○ Misalignment extinguishes this effect
▪

Holistic processing
- Faces are unlike other objects because they are holistically processed

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 - Faces are unlike other objects because they are holistically processed
○ All the features of the face are processed together, not one at a time
○ A face is perceived as a complete whole, not a sum of parts
Clinical evidence for holistic/specialised face processing
- The part-whole effect
○ subjects are better at distinguishing between two slightly different face parts (e.g. noses) in the context of a whole face t han in isolation or in
a scrambled face
▪ Demonstrates holistic processing in upright and intact faces
▪ Demonstrates individual processing of elements in scrambled and inverted faces
○ This phenomenon does not occur when participants are presented with other objects that are inverted/scrambled etc.
- Visual agnosia and double dissociation
○ The existence of two distinct systems/types of visual agnosia lends itself to supporting the holistic processing of faces
▪ Part based agnosia, depends on the ability to recognise parts, leads to an inability to recognise objects
▪ Holistic agnosia, depends on the configuration of "parts", leads to an inability to recognise faces
○ Double dissociation between face and object processing
▪ The fact that people with prosopagnosia (face blindness) often have little impairment in object recognition and vice versa indicates that
facial recognition does not depend on the object recognition pathway
□ *only when faces are not scrambled/inverted, then the object recognition pathway is used
□ Object recognition of external features (ears, hair etc.) can also aid in recognition
○ However, Intact face recognition with severe agnosia are rare
▪ CK case, could recognise faces with no deficit but not any objects
▪ Implies that face recognition does not get output from a general object recognition process
Tests of facial recognition
BTWF
- Before They Were Famous (BTWF) test
○ 59 pictures of celebrities, many of the photos taken when the people were children
○ correct identification of adult face requires generalization across substantial change in the appearance of the face
○ Does depend on prior exposure/culture
Cambridge face test
- Instructs you to learn faces and then identify them in novel images alongside distractor items
○ Relies on memory, not facial perception
○ Short form and long form, less vs. more difficult

Cambridge face perception test

- Doesn't rely on memory
- Requires you to order photographs based on their similarity to an example face provided

Super-recognisers and developmental prosopagnosia

- The existence of both super recogniser (who never forget a face) and those with developmental prosopagnosia (inability/extreme difficult
remembering faces since birth) indicates that facial recognition abilities lie across a spectrum
○ A normative model
Neural evidence for specialised face processing regions
- The hallmark of neural processing is adaptation
○ Adaptation is when our brains habituate to a certain stimuli, and prolonged exposure causes changes in our neural response
▪ e.g. seeing an after image
- Adaptation exists for faces
Our perception of faces is altered by prolonged exposure

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 ○ Our perception of faces is altered by prolonged exposure
▪ This implies that there are neurons that exist that are specific to face processing, because their continued firing creates adaptation
effects due to prolonged exposure
○ e.g. if you stare at the collection of red dots for a prolonged period of time, you brain will adapt to taking in the visual information of the two
faces at once, then if you stare at the red dot between the two composite faces, one will look more like Clinton and the othe r more like Bush
because your brain has adapted to the prolonged visual stimuli of the top two faces

Evidence for face neurons from non-human studies

- Using non-human primates
○ Monkeys, mostly macaque
- Monkeys are a good model for human brains because
○ They process human faces well
○ They are also social animals, so require the same evolutionary ability to recognise in/out group members and family members
○ Macaques have functional organization of early visual areas nearly identical to human
▪ Their brains are very much like ours
- Research has included single cell output recordings, fMRI and microsimulation
Kobatake & Tanaka (1994)
- Found face specific cells in the inferior temporal lobe (IT lobe)
○ Used extracellular single-cell recordings to pick up the action potentials of neurons
- Cells in this particular part of the IT responded best to whole faces, but not to parts of faces or incomplete faces
○ Faces must have at least eyes and mouth and be encapsulated in a circle with the correct and expected contrast

Tsao et al. (2006)

- Used fMRI
○ Showed monkeys 96 images of faces, bodies, fruits, technological gadgets, hands, and scrambled pixel patterns

- Found an area of 500 neurons that responded only to faces and nothing else in the IT (inferior temporal lobe)
○ 97% of visually responsive cells responded more to faces, with most only responding to faces
- The data indicated that specific areas of the brain responded to intact faces, which were completely separate from the areas that responded to
objects

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 objects

Afraz et al. (2006)

- Used micro-stimulation, and electrode capable of stimulating small groups of IT neurons
- Stimulation to the IT ( inferior temporal cortex) areas biases whether a primate sees a face or a flower
○ In both purely ambiguous and less ambiguous situations
- Experimental setup
○ Monkeys were presented with noisy images of either a flower or a face that had different noise levels
▪ Noise meaning pixelization
○ Monkeys were trained to look towards either a flower or face on target presentation depending on which they perceived in the first
presentation
○ When the images were presented, the IT was stimulated at different points in time in each different condition

- Stimulation made the monkeys more likely to see a face at all levels of noise in the picture (graph above)
○ 50-100 ms worked best
○ 60% of pure noise samples with stimulation at 50-100 ms reported perceiving a face, which indicated that stimulating the IT biases you
towards perceiving a face
Evidence in humans
- Uses fMRI, ECoG and stimulation
- Human brain face areas vs macaque brain face areas

Kanwisher et al. (1997)

- Discovered and named the FFA (fusiform face area)
○ activated specifically by whole faces
- The FFA does not respond differently when
○ presented with faces that have had features swapped out for another with small changes (e.g. different noses)
○ the spacing of the different features is changed (e.g. the space between eyes)
- Another brain area called the LOC responds to changes in features and spacing (Kanwisher & Yovel (2006))

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 Another brain area called the LOC responds to changes in features and spacing (Kanwisher & Yovel (2006))

Elbich & Scherf (2017)

- Investigated face processing areas outside of the FFA
- Found that your face recognition ability and aptitude was related to the amount and size of activation in the face selective areas of the fusiform
gyrus

- Activity in the right amygdala and the anterior temporal lobe was also related to your face recognition ability

- The more of the facial recognition network is activated, the better your recognition skills are

Quiroga et al. (2005)

- ECoG study with epilepsy patients
- There are cells that respond to specific individuals identities
○ Respond to face and textual representation of their names

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 ○

Jacques et al. (2016)

- Compared fMRI results to ECoG results for the same participants
- Measured category-selective responses in human ventral temporal cortex using ECoG arrays
○ Pre surgery epilepsy patients
- Strongest positive correlation between fMRI responses and ECoG signals in the High Frequency Broadband (HFB) of the ECoG (30–160 Hz)
○ fMRI and ECoG correlation stronger in spatial arrangement for faces than houses
Rangarajan (2014)
- Electrical Stimulation of fusiform gyrus alters face perception
○ Used a combination of ECoG and EBS (electrical brain stimulation using the ECoG arrays)
- Found that
○ left & right Fusiform Gyrus showed face-selectivity in the HFB
○ EBS of the right FG caused distortions of faces
▪ EBS of the left FG led to nonspecific visual changes like colour distortions
○ Stimulation of sites that caused face-related changes were, on average, more face-selective

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FFA specialization
Monday, 6 March 2023 6:02 PM

- The FFA might not represent an area that is specialized in face processing, but rather represents an area that is used when we process an object in
an area we have expertise in
- The competing theories of face processing are
○ Domain specific theory
▪ The FFA processes faces and faces only
○ Expertise hypothesis
▪ The FFA activity represents the processing of visual stimuli we have expertise in
▪ Does not just have to be faces, could be any stimuli
Evidence for the FFA processing faces only
- The face inversion effect is strong evidence for the domain specific theory
○ It concludes that upright faces are processed holistically, which sets faces apart from other objects
- Face selectivity in the FFA in people without sight
○ Both Blind and sighted people have activation of their FFA when feeling models of faces
○ Implications
▪ Perceptual expertise is not necessary for the development of face-selective responses in the FFA
▪ visual experience is not necessary for the development of face-selective responses in the FFA
▪ Connections from multiple brain regions may play a role in the development of cortical specificity in the FFA
Evidences for expertise causing inversion effects
Diamond & Carey (1986)
- Found that inversion effects exist for stimuli other than faces
○ Landscape pictures, only a modest effect

- Also found inversion effect for dog silhouettes in dog experts

○ Dog novices did not display an inversion effect

- This has been taken as evidence that maybe expertise is responsible for the inversion effect
- However, there has been no replication that has found the same results since 1986
○ e.g. Robbins et al. (2007)

Rossion & Curran (2010)

- Found that inversion effects for cars are related to level of car expertise
○ They tested the expertise of their participants first
- They found that experts in cars has an inversion effect for cars but not faces, and novices had the opposite (faces but not cars)
Operationalized as the cost of inversion, how much slower you were in the inverted condition

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 ○ Operationalized as the cost of inversion, how much slower you were in the inverted condition

Gauthier et al. (2000)

- Found greater activation in the FFA when looking at objects of expertise
○ Different responses across image categories (car/bird etc) but response in the FFA is always greater for expertise objects compared to non-
expertise objects

- However, its not ideal trying to study the effect of expertise on FFA responses using stimuli we are familiar with
○ We can’t quantify the degree of learning or familiarity with naturally occurring objects e.g. birds or dogs
Gauthier & Tarr (1997)
- Invention of Greebles
○ Unique visual stimuli that had never been seen before
○ Greebles have family, gender, and individual levels of classification

- This study trained people to become greeble experts

○ Expertise was recognized as someone who could identify a Greeble at the subordinate (individual) level as quickly as the sameGreeble’s gender
and family
○ Expertise was achieved on average in 3240 trials

- An fMRI study (Gauthier & Tarr, 1999) showed that there were small amounts of activation in the FFA in greeble experts when identifying greebles

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 ○

- Training leads to the “whole part” effect in Greeble experts but only in upright configurations
○ subjects are better at distinguishing between two slightly different greeble parts in the context of a whole greeble than in isolation or in a
scrambled greeble
○ These effects are not present in novice data
- Greeble expert recognition behavior and the physiology seem to be consistent with that found for face experts
○ Leads to the conclusion that the FFA is for expertise
Criticism of Gauthier by Kanwisher (2006)
- ''experts'' received to little training
- The response to greebles is the FFA is much less than the response to faces
- Greebles resemble faces
○ Greebles results differ from studies using objects that do not resemble faces/bodies

Prosopagnosia and greeble expertise

- Duchaine et al. (2004) discovered that prosopagnosics can be greeble experts
○ Indicating that they are processed differently to faces
- Edward (age 53) reports lifelong difficulties with face recognition but no serious head trauma and has 2 PhDs in physics and theology
○ No medical reasons, otherwise very smart
- No difficulty with object recognition but face recognition is very poor
○ > 6 sd below mean

- Edward has excellent Greeble recognition

○ Indicates face deficits do not involve brain processes used to acquire Greeble expertise

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Expression perception
Thursday, 9 March 2023 4:58 PM

- Expression recognition is important as social animals

○ Ability to decode behavioural intent is beneficial and socially adaptive
- The decoding of facial expression is innate
○ It is present in newborns and young infants
○ Emotional expressions still evident in deaf-blind people, congenitally deaf-blind have the same basic repertoire of spontaneous facial expression as
“normal” controls (Eibesfeldt, 1973)
- Facial emotion expression is similar across cultures
○ 6 universally recognised expressions (Ekman & Friesen, 1976):
▪ Happiness
▪ Surprise
▪ Fear
▪ Sadness
▪ Disgust
▪ Anger
- Facial expressions are processed holistically
○ expression judgements for composite faces are unaffected by their component faces' identities, and vice versa
- Facial recognition and expression perception are independent from each other
Prosopagnosia and expression recognition
- Most prosopagnosics can recognise facial expressions with no difficulty
○ Indicates that facial recognition and expression recognition are two separate processes
○ However, some developmental prosopagnosics do have deficits in recognizing facial expressions
- Prosopagnosics have impaired holistic processing for emotion
○ They use singular features like the mouth to decode emotions
- Prosopagnosics have impaired processing for emotion using eyes
○ diminished ability to classify facial expressions using only the eye-region

Anger superiority effect

- Threatening faces have a 'pop out' effect
○ There seems to be a pre-attentive search for threat
○ Detecting threat is very important evolutionarily for survival
- The amygdala may guide attention to emotionally relevant information (Jacobs et al., 2012)
Feature salience
- However, the superiority effect may reflect feature salience
○ Some people find an advantage for happy faces (Becker et al., 2011)
○ Although it is suggested that the salience of features like bared teeth cause pop out effect
○ Therefore, open and closed mouths may lead to the superiority effect (Horstmann, Lipp & Becker, 2012)

Facial Action Coding

- Decoding expressions by recognising which muscles the face is using
○ Idea originally Hjorztsjö (1969), developed by Ekman & Friesen (1978)

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 ○

Perception of emotions over time

- Emotional expressions are not static, they change over the time we perceive them
○ e.g. changes in muscle activity and tone
- Example of the dynamic changes in facial expressions over time (Jack et al., 2014)
○ Anger is more quickly apparent than happiness

- The activation of brain areas for processing expressions changes across the time we view expressions
○ (Adolphs, 2002)

- Research indicates we process emotional content in faces automatically, whereas attending to the emotional content and proces sing it consciously
happens later
○ 90ms orbito-frontal response to emotion regardless of attention
○ 170ms right insula response when attending to emotion
○ 220ms activation increase with identity processing
Role of the amygdala
- Suggested that the amygdala may help guide attention to emotionally relevant information
- The loss/damage of the amygdala causes deficits in recognising fear responses in others
○ It also responds to other emotions (Whalen, 2013)
▪ Anger
▪ Disgust
▪ Sadness
▪ happiness
- The amygdala is more activated by threatening faces over threatening images
○ Preferential right amygdala response (Hariri et al., 2002)
- It is also especially activated by eyes
○ Fearful/surprised eyes (with large whites) > normal eyes
○ does respond more to whole faces
- Amygdala is active in emotional decision making
Patient SM
- She had Urbach-Wiethe disease: lead to calcification of the amygdala
○ Results in a rare bilateral amygdala lesion
○ SM lost amygdala functioning at 10 years old
- Impairments
○ Does not recognise facial emotion in others
▪ She displays an absence of fixations on the eyes
▪ she looks at the mouth rather than the eyes when talking
▪ Can fixate on the eyes only when directed to, then she can accurately process facial emotion
□ Suggests that the amygdala is important for initiating eye movements for emotional decision-making

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 ▪

○ Lack of natural fear to snakes and spiders

○ lack of loss aversion (gambling)
- Preserved ability
○ Does perceive fearful tones in voice
○ Does experience fear from images of body positions
○ Has a normal startle response
▪ a sudden loud boom or flash of light will evoke an automatic response
▪ Suggests a neural pathway independent of the amygdala for startle responses

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Blindsight
Monday, 3 April 2023 7:07 PM

- Hemianopia
○ partial or total cortical blindness caused by destruction of the primary visual cortex (V1)
○ Usually only unilateral vision loss, but can be bilateral

○ Clinical signs of blindness

▪ No visual response to threat
▪ Optokinetic reflex may be absent
□ When an individual tracks a moving object with their eyes, which then moves out of the field of vision,
then their eyes move back to the initial position when they first saw the object
▪ No alpha rhythm (EEG) on posterior regions
▪ Absence of visual evoked potentials (ERP)
- Blindsight
○ An ability that people with lesions to the primary visual cortex have to perceive objects even though they can't
consciously see or be aware of visual stimuli
▪ Report no awareness of visual information
○ Yet there has been evidence since the 70s that some people can still perceive stimuli
▪ Poppel et al., Nature (1973) observed that war veterans could orient gaze towards (unseen) stimuli in the blind
field
▪ Humphrey, Perception (1974) shows that a blind monkey retains spatial navigation abilities
- Preserved abilities include
○ Orienting gaze towards stimuli shown in blind field
○ Point to stimuli location when shown in blind field
○ Guess object orientation well above chance
○ Identify letters in forced choice paradigms well above chance
○ Guess the presence of movement above chance
○ Distinguish between shapes above chance
○ Preserved navigation skills, navigating around obstacles
Pathways of visual information in blindsight
- Regular pathway for visual information is the geniculo-striate pathway
○ Retina -> LGN -> V1 etc.

- In the event of damage, action potentials must find alternative pathways

○ Colliculus pathway
▪ Retina -> superior colliculus (SC) -> pulvinar -> V2
○ LGN bridge
▪ Retina -> LGN -> V5
○ Pulvinar pathway
▪ Retina -> pulvinar -> V5 (V5 is a motion sensor)
Colliculus pathway evidence

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Colliculus pathway evidence
○ Retina -> superior colliculus (SC) -> pulvinar -> V2
- Evidence 1- Monkeys lose blindsight after SC (superior colliculus) destruction
○ Suggests colliculus pathway is at least partially responsible
- Evidence 2 - Distractors in the blind temporal field (not nasal) delay saccades to targets displayed on the retained visual field
○ Temporal retina projects to the superior colliculus, but the nasal field which project directly to V1
▪ Nasal retina, the half of the retina closest to the nose
□ Nasal retina -> across the optic chiasm -> LGN -> V1
□ Therefore this pathway is impaired when there is damage to V1
▪ Temporal retina, the half of the retina closest to the temporal lobe
□ Temporal retina -> SC -> pulvinar -> V2 etc
□ This pathway is not impaired with V1 damage, hence the saccade delay when distractors are projected
onto the temporal retina

Evidence contrary to the colliculus pathway

- fMRI study, Zeki and Ffytche, Brain 1998
- Activation in V5 in response to moving stimuli but no colliculus response
○ If the colliculus pathway was correct, there would have to be a colliculus response
○ It is hard to draw definitive conclusions based on fMRI, it is all statistical mapping, so there may have been effects that
weren't observed
LGN pathway evidence
- White matter integrity investigation
○ Ajina et al., eLife 2015, Compared intact white matter pathways to damaged white matter pathways between the
LGN and V5
○ Evidence suggests that blindsight information is travelling through the LGN to V5
▪ Compared blindsight positive and negative patients with unilateral V1 lesions
▪ They found that the integrity of pathways between the LGN and V5 was far more similar between injured and
non-injured hemispheres in patients with blindsight than patients without blindsight
□ Integrity means that the myelination is strong, water molecules are being channelled along these
pathways efficiently
▪ They also found that the diffusivity of water molecules (non-integrity) was far higher in the damaged
hemispheres of non-blindsight patients
□ the diffusivity in blindsight patients in both hemispheres was similar to that of the non-damaged
hemispheres in non-blindsight patients
 Fractional anisotropy = good integrity, mean diffusivity = bad integrity

- Reversable LGN lesions in monkeys with damaged V1 who display blindsight

○ Reversable lesions stop the LGN from working temporarily
○ When the reversible LGN lesion is applied, the activity in the visual areas significantly decreases compared to when
monkeys only have a V1 lesion
▪ Suggests that in blindsight monkeys, the LGN is transmitting visual information to other visual areas

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 ▪ Suggests that in blindsight monkeys, the LGN is transmitting visual information to other visual areas
○ The LGN lesion temporarily removes blindsight abilities
○ Lesion/Les = V1 lesion, Inj = LGN lesion

Pulvinar pathway evidence

- White matter integrity
○ Strengthened pathways between the pulvinar and the V5 in blindsight patients blind from birth compared to controls

- The pulvinar pathways may be utilised in patients who were cortically blind since birth/early infancy
○ This is because before brain development post birth, there are lots of connections between the pulvinar and the V5
that get pruned with age
▪ But if you are cortically blind, you may instead strengthen these pathways instead of the usual V1 pathways
○ If the damage occurs later in life the LGN takes over

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Affective blindsight
Saturday, 8 April 2023 9:38 AM

- People with affective blindsight can still perceive emotions via facial expressions
○ Can effectively distinguish between expressions

- In patients with unilateral hemianopia, the amygdala will respond to emotional faces
○ The left amygdala responds to both seen and unseen fear, and the right amygdala responds to unseen fear,
○ visual areas only respond to consciously seen stimuli
▪ Consciously processed in left and unconsciously processed in right
▪ Morris et al., 2001

- Unseen happy body images selectively activate the V5 and the pulvinar in hemianopia, without activating visual areas
○ de Gelder and Nouchine, 2006
- Patients with hemianopia can experience fear conditioning to a visual cue presented in their unseen visual areas
○ Unseen visual cues paired with loud noises
○ Indicates that fear conditioning does not require conscious/visual awareness
Mechanism of affective blindsight
- Retina -> SC -> pulvinar -> amygdala

Diffusion transmission imaging DTI

- Tracing the integrity of white matter pathways in the brain
○ Diffusion of water molecules along white matter fibres
○ Fractional anisotropy (good pathway = high) and mean diffusivity (bad pathway = high)
- DTI shows connectivity between SC and the amygdala in humans and macaque
○ Rafal et al., 2015
- Patient GY
○ lesions in left V1, but intact right V1
○ shows normal SC-Pulv-AMG connectivity (via DTI) on the intact side but increased connectivity on the lesioned side
○ Using fractional anisotropy where higher = greater white matter integrity
▪ Indicates that on the left side there is compensatory connections between the SC-Pulv-AMG pathways

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 ▪ Indicates that on the left side there is compensatory connections between the SC-Pulv-AMG pathways

- Pathways between SC-Pulv-AMG found in control samples

○ The density of these pathways positively predicts fear recognition in faces
○ McFadyen, Mattingley, Garrido., 2019
Dynamic causality modelling
- How the dynamics of neural activity in one area relate to the dynamics in another area
○ Mapping functional connectivity
○ How well activity in one area predicts activity in another
- Greater White Matter Connectivity Between Right Pulvinar And Amygdala Is Correlated With Stronger Functional Connectivity
○ There is an association between Pulv-AMG connections and how fear impacts and changes activity in the amygdala and
pulvinar

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Spatial neglect
Monday, 17 April 2023 6:31 PM

- Patients with hemispatial neglect are not consciously aware of one side of their world
○ Problem with attention rather than vision
▪ They can see it if they are told to attend to it, but without prompting they will neglect to attend to one side
○ e.g. only eat food on one side of the plate, only shave half of their face, only copy one half of an image
- Occurs in stroke survivors
○ Usually lesions in the right parietal lobe in the right angular gyrus
▪ Neglect is less severe and demonstrates faster recovery for left hemisphere lesions (right side neglect)

- One lesion will cause contralesional neglect

○ Neglect in the area of vision contralateral to the lesion (usually the left side of vision for right hemisphere lesions)
- Patients will spontaneously orient to their preserved side
○ Ipsilesional orientation
- Most neglect cases concerns near space neglect rather than far space
○ Some patients have the opposite
- Neglect patients have right hemisphere dominance for processing stimuli
○ Compensatory left parieto-frontal connectivity reduces neglect severity
- Pseudo neglect and spatial asymmetry is present in the nonclinical population
○ Line bisection tasks show a slight left favouritism effect
○ This is associated with larger right superior longitudinal fasciculus II volume
▪ A larger volume causes greater deviation
- Neglect does not only affect vision, it can also affect auditory stimuli and attentional processes
Clinical assessment of hemispatial neglect
- Drawing
○ Spontaneous drawing or coping off a model
○ Patients often neglect to draw one half of the picture
- Line bisection tasks
○ Patients will bisect the line further towards the right side
○ How much further towards the right indicates how bad the spatial neglect is
- Cancelation of line tasks
○ Will not cross out lines on the neglected side of vision
- Clock drawings
○ Will neglect to draw half of the clock
- Spatial extinction task
○ Proves that neglect patients can see but favour the preserved side when prompted with competing stimuli

Rehabilitation
- Spontaneous remission can occur

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 - Spontaneous remission can occur
- Optical wedge prism glasses
○ Glasses with large lenses that displace the visual field further towards the right
○ This causes over correction towards the left and perception of more of the left visual field

- Outcomes include more complete perception of the left visual field

○ Complete drawings, line cancelation tasks and drawings from memory better

- There is a lasting effect after prism adaptation

○ Brain changes in the right posterior cerebellum (associated with both spatial realignment and error correction), right
inferior parietal lobe (IPL) angular gyrus (spatial realignment) and right anterior PL (associated with spatial realignment
and error correction)

Attentional difficulties
- Working memory is impaired in neglect patients
○ In search tasks, there is a deficit in retaining searched locations

- Attentional blink is more severe in hemispatial neglect patients

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 - Attentional blink is more severe in hemispatial neglect patients
○ Accuracy in the task requires a much longer period between target for participants to notice them both
- The indicates that neglect causes issues with attention over time, as well as spatial issues

Neglect and blindsight

- Neglect patients have been shown to still have some capacity for processing stimuli on the neglected side
○ e.g. the house on fire experiment
▪ The houses are perceived as the same (confirmed verbally) but in 9/11 trials participants chose to live in the
bottom house

○ Patients can also be primed to perceived stimuli on the right (retained side) by stimuli presented on the left (neglected
side)
▪ Indicates that visual information is being unconsciously processed

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Colour perception
Friday, 12 May 2023 2:59 PM

- Our current understanding of the visual system does not explain the vast distribution of experiences that exist in terms of
colour vision
Krauskopf et al. (1982)
- Investigating the mechanisms of colour perception
- Participants required to indicate whether a dot was visible on a similarly coloured background before and after prolonged
exposure (30 secs + 5 secs) to a similar stimulus modulated along the spherical spectrum of colour
○ Investigating how habituation affected present/absent decisions
▪ Habituation screens of cardinal colours
□ Blueish-yellowish and reddish-greenish, mixed spectrum and not the same as named colours
□ Simply the presence or absence of general hues

○ The spectrum of colour can be represented by a sphere with different attributes of the colour varying along an X, Y and
Z axis of blue/yellow, red/green and black/white

- The results of these experiments indicate that blue-yellow, red-green and black-white decisions are processed independently
○ strongly suggests three independent detection mechanisms mediate the transmission of spatio-chromatic information
from retina to cortex
○ Dots of colours part of the blue-yellow axis were less likely to be detected after habituation to a blue-yellow
habituation screen
- Colour that include both blue-yellow and red-green elements are equally affected by habituation to each spectrum

○ This demonstrates that when you are shown an adaptation stimuli of blue-yellow and then shown a you are then
shown a blue dot on a blue background, you require greater contrast between the two blues to be able to distinguish
between them after being exposed to the adaptation stimuli then when backgrounds/dots of other colours not on the
blue-yellow spectrum
▪ The more red-green that is present in the colour, the less contrast needed to detect the dot
Neuronal representations of colour
- LGN neurons respond to one of the three different spectra of colours

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 - LGN neurons respond to one of the three different spectra of colours
▪ Parvocellular layer responds to red/green (and also luminance)
▪ Koniocellular layer responds to blue/yellow
▪ Magnocellular layer responds to black/white
○ However, LGN neurons do not habituate to prolonged exposure
- Cortex neurons represent more complicated colour profiles
○ They receive input from all types of LGN neurons
○ Cortical neurons adapt to stimuli
- In order to explain both colour perception and adaptation, both the cortex and LGN neurons must be taken into account
○ There is no one localised group of neurons that coordinate the entirety of colour perception
▪ Cardinal neurons do not exist
○ Furthermore, there is a feedback loop between the cortex and the LGN
▪ There are more neural pathways cortex -> LGN than the other way around
▪ Perhaps it is the cardinal system?
- This suggests that even for the most simple processes, such as vision, many different groups of neurons are required, with
each group coordinating a separate aspect of the process
Colour perception sensitivity
- Human colour vision is a property of the visual system, not due to the characteristics of single neurons
○ Colour sensitivity (same/different decisions) is much greater than the sensitivity that could be facilitated by
modulations to the excitation and inhibition of one neuron
○ Our colour vision is 10 times more sensitive to colour variations than the sensitivity of single neurons that receive
information from groups of cones
▪ Our sensitivity to colour variation is better than the sensitivity of neurons to input from cone cells
- The anatomical structure of the pathways from the retina to the brain indicate that our ability to perceive colour stems from
both the cone cells and the way the structure of our brain is arranged when processing those signals from the cones
○ There are two major pathways from the LGN to the Cortex for visual information after it arrives from the retina
▪ The M pathway, which is mostly for luminance sensitivity
□ There is no expansion of neural pathways between the retina -> LGN, but expansion between LGN -> cortex
▪ The P pathway, which is more colour selective
□ There is retinal-cortical expansion between the retina -> LGN in terms of neuron : neuron input ratio
□ This facilitates the cortex-LGN feedback pathway
○ Therefore, feedback modulates the parvocellular colour processing pathway but not the magnocellular pathway

- Colour and motion processing

○ When viewing motion, movement must be coded and perceived very quickly
▪ Motion perception has its own part of the brain that is specialised for motion processing
○ For colour the opposite is true, the feedback loop for perceiving colour takes times to determine what you are seeing
▪ Therefore, viewing moving chromatic stimuli can cause weird perceptions
▪ You are not going to get a good sense of motion when perceiving moving chromatic stimuli, moving chromatic
stimuli look slower than they actually are because of the colour processing feedback loop slowing down
perception
○ there is a purely chromatic motion mechanism but that it is limited to the fovea
▪ there are at least two different mechanisms for motion stimuli, one with colour and one without colour
- Cardinality and neural networks
○ It appears that networks of neurons in the V1 and V2 act as cardinal networks
However, the neurons themselves are not cardinal neurons

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 ○ However, the neurons themselves are not cardinal neurons
- Colour categorisation is not the same as discrimination
○ Discrimination between colours can occur within one category

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